Perinatal exposure to toxic substances can cause long-lasting behavioral and neuronal effects. Neonatal rats exposed to a severe anoxic episode from inhalation of 0.4% carbon monoxide for 1 to 2 hrs are hyperactive as young rats but recover as adults. Caudate neurons in these rats show progressive changes in numbers of dendritic spines consistent with the hypothesis that plasticty in caudate neuronal morphology is associated with changes in locomotor activity. These studies are being extended to exposure of rats in utero to carbon monoxide on gestational day 15. Activity is being recorded in the post-weaning period. Several additional morphometric measurements have been used to detect caudate neuronal damage after fetal exposure to x-irradiation (125 r) on gestational day 15. These are measurements of soma size, neuronal size and dendritic branching patterns. All 4 parameters are reduced in caudate neurons of x-irradiated rats. Caudate neurons and locomotor activity of offspring exposed to carbon monoxide on gestational day 15 are being compared with the same measurements in rats exposed to gestational x-irradiation.